Gas turbine engines draw air into the engine using a compressor. Fuel is mixed with the air and combustion takes place. The hot exhaust gasses pass through a turbine to drive the compressor and are exhausted out the back of the engine to drive the airplane. The turbine includes a large number of turbine blades that rotate when the hot gas impacts them. A shroud surrounds the blades to direct the hot gasses on to the blades and prevent leakage of the gas.
The clearance between the blades and the shroud is kept to a minimum during ordinary or steady state operation of the turbine. During take-off and reverse thrust, the blades will contact the shroud. The shroud is made of an abradable material in order to minimize damage to the turbine blades. At periodic intervals, the shroud is repaired by coating with an alloy on its inner surface to re-establish the proper clearance for the shroud and turbine blades.
In recent years, the shroud has been made from nickel-based superalloys rather than cobalt-based superalloys because they operate at higher temperatures due to the microstructure of the alloy. In the past, cobalt-based superalloys have been found to be incompatible with nickel-based shrouds, and thus nickel-based superalloy coatings have been used with the nickel-based superalloy shrouds.